Inorganic oxide aerogels, especially of metal oxides, are well known in the state of the art in relation to a good number of applications. For example they can be used as catalysts, catalyst supports, adsorption means, chromatographic column packing, pigments, opacifiers, abrasives and for other purposes.
Examples of their use as catalysts or catalyst supports are given in U.S. Pat. No. 3,673,111 (preparation of phosphoric acids) and in the article in "Journal of Molecular Catalysis", Vol. 17, 219-223 (1983) (radium catalyst support in the preparation of olefins from CO and H.sub.2). They are also used as catalyst supports in the preparation of phthalic anhydride and the decomposition of hydrogen peroxide.
Normally the known powders, and particularly those used as catalyst supports, consist of inorganic oxides obtained by precipitating hydroxides from aqueous solutions of the corresponding cations, followed by calcining.
Powders obtained in this manner generally have a very low specific surface and porosity, the spherical particles do not always possess suitable mechanical properties, and their particle size distribution is very wide, which sometimes discourages their use for the intended application. In this respect it is not infrequently the case that catalytic systems obtained using such powders do not enable the required reactions to be conducted with good productivity, which means that increasing catalyst quantities have to be used, requiring subsequent costly wash processes for the catalyst residues.
Various methods of preparing aerogels are known, independently of their final use.
For example the article in "Advances in Colloid and Interface Science", 5 (1976) 245-273 describes a general method for preparing SiO.sub.2, Al.sub.2 O.sub.3, MgO and other aerogels by hydrolysis and polycondensation of the alcoholate of the metal concerned, followed by drying under supercritical conditions. However with this method it is not possible to control the size distribution of the spherical particles and maintain a large percentage within a narrow pore diameter range.
European patent 213,987 relates to catalyst supports in the form of spheroidal silica particles obtained by granulating an SiO.sub.2 hydrosol with an SiO.sub.2 aerogel and drying. The surface area of the product obtained never exceeds 500 m.sup.2 /g.
U.S. Pat. No. 4,246,137 describes a method for preparing silica-zirconia xerogels by reacting a zirconium compound with an alkaline metal silicate, ageing the hydrogel obtained, washing and finally drying. The product obtained in this manner has a total pore volume of between 1.5 and 3.5 ml/g, a surface area of between 200 and 600 m.sup.2 /g, but a pore diameter range of 200-600 .ANG..